Particulate adhesive containing polyepoxides carboxylated butadiene-acrylonitrile copolymer and a urea derivative as a curing agent

ABSTRACT

A CURABLE STRUCTURAL ADHESIVE IN FLOWABLE, PARTICULATE FORM COMPRISING AN EPOXY RESIN MODIFIED WITH A NITRILE RUBBER PREFERABLY CONTAINING FREE CARBOXYL GROUPS, AND AN EPOXY CURING SYSTEM WHICH PREFERABLY WILL PROVIDE A CURE AT A TEMPERATURE NOT EXCEEDING ABOUT 250*F. UPON SUBJECTION TO ABOUT 150*F., THE ADHESIVE CONVERTS TO A STATE IN WHICH IT WILL READILY ADHERE TO VARIOUS SUBSTRATES INCLUDING METALS. ALSO PROVIDED ARE A PROCESS FOR APPLYING THE ADHESIVE AS WELL AS THE SUBSTRATES CONTAINING THE ADHESIVE BOTH IN THE FUSED, NON-CURED STATE AND THE CURED STATE.

3,655,818 POLYEPOXIDES, CARBO ATED OPOLYMER,

GEN

AND A URE A 'I' 68 A. G. M KOWN PARTICULATE ADHESIVE CONTAI RYLONITRILNING E C D VATIVE AS A CURING Filed Oct. 18, 19

April 11, 1972 BUTADIENE INV ALA/v G. McKow/v BY M (@fi/M ATTORNEYSUnited States Patent @fioe Patented Apr. 11, 1972 PARTICULATE ADHESIVECONTAINING POLY- EPOXIDES, CARBOXYLATED BUTADIENE-AC- RYLONITRILECOPOLYMER, AND A UREA DE- RIVATIVE AS A CURING AGENT Alan G. McKown, St.Paul, Minn., assignor to Minnesota Mining and Manufacturing Company, St.Paul, Minn.

Filed Oct. 18, 1968, Ser. No. 768,675 Int. Cl. C08g 45/04 US. Cl.260-837 14 Claims ABSTRACT OF THE DISCLOSURE A curable structuraladhesive in flowable, particulate form comprising an epoxy resinmodified with a nitrile rubber preferably containing free carboxylgroups, and an epoxy curing system which preferably will provide a cureat a temperature not exceeding about 250 F. Upon subjection to about 150F., the adhesive converts to a state in which it will readily adhere tovarious substrates including metals. Also provided are a process forapplying the adhesive as well as the substrates containing the adhesiveboth in the fused, non-cured state and the cured state.

This invention relates to structural adhesives; more particularly itrelates to one part epoxy based structural adhesives in flowable,particulate form especially adapted for adhering metal sheeting toitself and to honeycomb cores such as are employed in aircraft wingassemblies.

From a historical viewpoint, bonding of an airframe is a recentdevelopment. The first production usage of a bonded structure was inWorld War II bombers. The upsurge of commercial aviation in the post-warperiod started the trend away from riveted assemblies towards a bondedassembly. The reasons involved were numerous but included cleaner airfoil surfaces, improved fatigue strength, and a greater strength toWeight ratio through the use of honeycomb construction.

Initially, bonding was confined to control surface area and empennagebut as improvements in adhesives were made, the use of adhesivesexpanded until today it includes a large percentage of the aircraftincluding wings, rotor blades and fuselage.

The development of film adhesives was a prime factor for this increasein usage as it offered adhesives in 100% solids form and permittedapplication of a controlled amount of adhesive. Other adhesivesavailable were solvent solutions where drying was a major difficulty or100% solids pastes which at least prior to the advent of films did notpossess the fine balance of properties required for a structuraladhesive. However, even with films various hand lay-up operations arerequired in making a bonded assembly including removing a protectiveliner, positioning the film, removing a second protective liner, makingcutouts in desired areas, and finally closing the bond. In addition toinvolving undue amounts of time, film adhesives suffered from otherdrawbacks including waste in material from cutting operations,difficulty in applying to contoured surfaces, inability to provideadjustments in thickness, and limitations in com pensating for mismatchof parts.

Adhesives in paste form are even less desirable due in part to theirtackiness at the time of application making precise positioning of thebonded elements difficult, difiiculty in controlling the amount andlocation of the adhesive, and the further fact that solvents arerequired to remove the adhesive from unwanted areas. Liquid solventcontaining structural adhesives also lacked suitability due to inabilityto localize application, the need for solvent removal with its attendantfire and toxicity hazards, and, as with the pastes, the tackiness of theadhesive at the time of application.

With the advent of the large jet aircraft the amount of bonded area perplane and the total number of planes anticipated will far outstrip theproduction facilities and work force available. It is not consideredeconomically sound to merely increase floor space and hire more labor.For the above reasons, the presently available adhesives, both in termsof composition and form, do not lend themselves to the automatedtechniques which must be employed to meet this expanding demand.

To achieve a structural adhesive amenable to high production techniquesand yet fulfill the stringent requirements of structural adhesives foraerospace environments is the primary object of this invention. Theserequirements are particularly formidable, calling for an adhesive whichexhibits excellent strength properties over a wide temperature rangewhich in turn depends on attainment of a delicate balance of suchproperties as adhesion, toughness, and tensile strength. The followingtable indicates the kind and value of properties over a wide temperaturerange desired in the bonds within such laminate structural members asare found in aircraft:

T-peel (minimum pounds/ ch of width) 15 15 Honeycomb peel 3 (minimum ininches of width) 10 10 10 Beam-creep 4 (maximum in mils) 50 50 1 Thefree ends of strips of 1 inch wide, 4 inches long, 64 mil 2024 T8 cladaluminum alloy sheeting, bonded together at their other ends with 0.08pound/square foot weight adhesive (used in all the tests) in a inchoverlapping joint, are pulled in opposite directions along theirlongitudinal axes.

2 The adjacent ends of 1 inch Wide, 8 inches long strips of 20 mil 20%T3 clad aluminum alloy sheeting adhered together over most of theirlength are bent apart at right angles and are pulled in oppositedirections.

f A free end of a 3 inches wide, 10 inches long, 20 mil 2024 T3 cladaluimnum alloy sheeting is pulled from the 6 inch thick, inch cellhoneycomb core of 4 mil 3003 aluminum alloy foil to which it is bondedby wrapping the sheeting around a 4-inch diameter roller riding on thesurface of the sheeting.

4 Three inches wide, 8 inches long laminates of the described honeycombcore with 64 mil skins of 2024 T3 clad aluminum alloy sheeting aresupported by supports spaced 6 inches apart. A 1,000 pound weight at F.and an 800 pound weight at F. are loaded midway between the suppors.After 192 hours the deformation of the center of the beam is measure Theachievement of such properties when bonding to honeycomb requires anadhesive that not only exhibits strength properties in the cured state,but that also, when first heated, has flow and other characteristicsnecessary to wet and form a fillet along the contacted edge portion ofthe honeycomb. Moreover, the presence of aluminum as the substrate inthe majority of aerospace structures to be bonded necessitatesimposition of a limitation in the curing temperature of the adhesive inorder to avoid reduction in corrosion and fatigue resistance of thealuminum. As a consequence, curing temperatures not exceeding about 250F. are highly desirable and in some cases necessary.

The present invention provides a structural adhesive possessing theabove stated properties as well as being amenable to automatedtechniques of application, such structural adhesive being a flowableparticulate comprising at least one heat curable epoxy resin having onthe average more than one 1,2 epoxy group per molecule, at least onecopolymer of butadiene and acrylonitrile, and a curing system for saidepoxy resin comprising at least one room temperature stable, nitrogencontaining compound decomposable to form at least one amine having atleast one active hydrogen atom.

Adhesives of the above composition and particulate form may be appliedby various automatic means including rollers, electrostatic sprayequipment, fluidized beds, and vibrating beds. In the completely uncuredstate, the adhesive is a fiowable particulate which can be readilyremoved from undesired areas by means of a vacuum tool. Upon subjectionto temperatures above about 120 F. and below the cure temperature,generally about 230- 250 F., the adhesive enters an agglomerated, fusedstate in which it adheres strongly to the substrate to which it isapplied and yet is not tacky or stick enough to cause individuallytreated substrates to stick together during storage or shipment.Moreover, in this state, the treated parts can be manipulated into thebonding position without the need for careful precautions to insureprecise initial matching. The adhesive can remain in this fusedadherable, non-tacky, curable state for periods of up to about 45 daysat temperatures less than about 90 F.

Because of this stability in an adherable state, it is now possible forthe manufacturer of the basic structural elements, e.g. the manufacturerof panels and honeycomb structures, to pre-coat such elements with theadhesive, selectively remove adhesive from undesired areas, heat theadhesive to a fused, adhering state, and ship the resulting product tothe ultimate fabricator such as the airplane manufacturer. Thus, theadhesive of this invention provides the opportunity for a form ofmarketing of structural units hitherto impossible with previousstructural adhesives, giving the ultimate manufacturer the option ofconcentrating on the final assembling techniques to WhlCh it is bestsuited.

Epoxy resins suitable in the practice of this invention arethermosettable polyethers having on the average more than one 1,2 epoxygroup per molecule, including the diglycidyl ethers of polyhydricphenols, glycidyl ethers of novolac resins, glycidyl ethers of aliphaticpolyols, and glycidyl ethers containing nitrogen. Preferred diglycidylethers of polyhydric phenols include the condensation product ofepichlorohydrin and Bisphenol A, especially those with an epoxyequivalent between about 550 and about 700 and a softening point betweenabout 75 C. and 85 C. Exemplary commercially available Bisphenol A typeepoxy resins are sold under the trade designations Epon 1002 (a soliddiglycidyl ether of Bisphenol A, epoxy equiv. wt. 600-700) and DER 662(a solid diglycidyl ether-Bisphenol-A resin, epoxy equiv. wt. 575-700).The glycidyl ethers of novolac resins are characterized by phenyl groupslinked by methylene bridges with epoxy groups pendant to the phenylgroups, commercially available resins being sold under the tradedesignation DEEN- 438 (a polyglycidyl ether of phenolformaldehydenovolac, epoxy equiv. wt. 176-18l) and ECN 1280 (a polyglycidyl ether oforthocresol-formaldehyde novolac, epox equiv. wt. 230). Comm'erciallyavailable glycidyl ethers of aliphatic polyols include those having thetrade designations ERL-4201 (3,4-epoxy 6 methylcyclohexylmethyl 3,4-epoxy-6-methylcyclohexane carboxylate, epoxy equiv. wt. 145-156) and ERL4289 (Bis(3,4-epoxy-6-methyl-cyclohexylmethyl)adipate, epoxy equiv.220'). A commercially available glycidyl ether containing nitrogen isERL '0510 (triglycidyl para-aminophenol, epoxy equiv. wt. 97- 101).

The epoxy resin may be solid or liquid so long as the overall adhesivecomposition constitutes a grindable mass, at least in the presence ofDry Ice, which will provide a powdered adhesive at room temperature.Some agglomeration of adhesive may occur at room temperature but theadhesive should be capable of being broken up into matter havingparticulate character. It is preferred tha at least some novolac epoxyresin be present to provide high temperature strength to the adhesive.Weight ratios of novolac epoxy to non-novolac epoxy resin of from 1:7 to3 :1 are suitable, with a major portion of novolac epoxy resin beingpreferred. If a liquid epoxy resin is employed, particularly a glycidylether of an aliphatic polyol or phenol, it should be combined with amajor portion of solid epoxy resin, preferably at least as much as 70%by Weight of the combined epoxy resin weight.

The nitrile rubbers-copolymers of butadiene and acrylonitrileserve asmodifiers for the epoxy resins. Preferably, the nitrile rubber containsa small percentage of carboxyl groups either terminally located ordistributed throughout the polymer chain or both. Nitrile rubbersderived from l8-46% acrylonitrile, 5582% butadiene, and 0-15 of acarboxylic acid represent typical formulations suitable in the practiceof this invention. Commercially available nitrile rubbers include thosesold under the trade designations Hycar 1072 (a carboxylatedacrylonitrile/butadiene (AN/BD) copolymer having an acrylo nitrilecontent of 30-35 by weight), Tylac 221A and Tylac 211A (a carboxylatedbutadiene/acrylonitrile rubbers having about 25% acrylonitrile and 5-10%carboxylic acid comonomer), Chemigum 550 (a carboxylated AN/BDcopolymer) and Hycar 1042 (a non-carboxylated AN/ BD copolymer having anacrylonitrile content of 32- 34% by weight). The preference forcarboxylated nitrile rubbers appears to be due to the reaction whichoccurs between the carboxyl groups and the epoxy groups which areactivated during the curing or crosslin'king reaction, thereby makingthe nitrile rubber an integral part of the cured system. While thisintegration is highly preferred, suitable adhesives have been providedwithout carboxylated nitrile rubbers.

The curing system for the epoxy resin includes at least one roomtemperature stable, nitrogen containing compound decomposable to provideat least one active hydrogen containing amine. Decomposition generallyoccurs at a temperature between F. and 250 F. to provide curing in thattemperature range generally within a period of one hour. For certainapplications, especially for bonding aluminum substrates in aerospacestructures, curing temperatures not exceeding 250 F. are desired. Forother substrates, curing systems providing a cure at about 350 F. aresuitable. Exemplary decomposable curing agents include monoandpoly-ureas, thioureas, and hydrazines, illustrative of which are thefollowing:

3-phenyl-l,1-dimethyl urea; 3-p-chlorophenyl-1,l-dimethyl urea;3-p-anisyl-1,1-dimethyl urea; 3-p-nitrophenyl-l,l-dimethyl urea;3-phenyl-l,l-cyclopentamethylene urea; 3-phenyl-1,1-cyclohexamethyleneurea; N-(3,4-dichlorophenyl)-N',N'-dimethyl urea; 3-phenyl-1,1-dibutylurea; 3-phenyl-l-benzyl-l-methyl urea; trimethylurea;

3-phenyl-1,1-dimethylene urea; 3-cyclohexyl-1,l-dimethyl urea;2,4-bis-(N,N-dimethyl carbamide) toluene; N,N'-dimethyl-1,3-propanediamine dicarboxanilide; 1,3-dicyclohexyl urea;

1,3-dimethylol urea;

1,3-diethyl thiourea;

thiourea;

urea;

3-phenyl-l,l-dimethyl thiourea; semicarbazide;

thiosemicarbazide;

4-phenyl-1,1-dim'ethyl semicarbazide; 4-phenyl-1,1-dimethylthiosemicarbazide; l-cyanoguanidine;

1,3-diphenyl guanidine; and 1,1'(4-methyl-m-phenylene)-bis-[3,3'-dimethylurea] Preferable among those listed are3-phenyl-1,1-dimethyl urea, 3-p-chlorophenyl-1,l-dimethyl urea, and3-p-nitrophenyl-1,1-dimethyl urea.

The active hydrogen containing amine decomposition product functions asa curing agent for the epoxy resin via an epoxy to epoxy crosslinkingreaction. Whereas curing can be elfectuated by the presence of suchdecomposition products alone, it has been found preferable to includecertain curing agents such as dicyandiamide and isophthalyldihydrazidewhich alone provide epoxy curing at about 325 F. It has been found thatsuch compounds in combination with the heat decomposable nitrogencontaining compounds, such as 3-(p-chlorophenyl)-1,1-dimethyl urea andthe like, provide a curative effect at temperatures as low as 190 F. Forsome reason not readily explainable the presence of these elevatedtemperature curing agents such as dicyandiamide andisophthalyldihydrazide contribute to the improvement of the physicalproperties of the cured system.

In addition to the room temperature stable, elevated temperaturedecomposable nitrogen containing compound, the curing system may contain(2) a hydroxyl containing organic compound and (3) an organo lead ormercury compound. Such a three-component curing system is disclosed inUS. application Ser. No. 644,797 assigned to the common assigne'e. Thiscuring system further reduces the cure temperature of the epoxy resinfrom about 250 F. to a temperature generally below 200 F.

The hydroxyl containing compound may be an aliphatic, alicyclic, oraromatic alcohol, carboxylic acid, hydroxy acid, or mixture thereof.Such compound may contain one or a plurality of hydroxy or carboxylgroups. Aliphatic polyhydroxy compounds are preferred, especiallyethylene glycol and glycerol. Representative hydroxyl containingcompounds are the following: ethylene glycol, glycerol, triethyleneglycol, Bisphenol A, methanol, n-butanol, phenol, o-cresol, m-cresol,p-cresol, resorcinol, o-bromophenol, n-hexanol, trichloracetic acid, andmixtures thereof.

Exemplary organo-mercury and organo-lead compounds are phenyl mercurichydroxide, phenyl mercuric acetate, phenyl mercuric stearate, leadoctoate, lead linoleate, and lead acetate. The organo-mercury andorgano-lead compounds, in combination with the nitrogen containingcompound and the above described hydroxyl containing compounds, providean unexpectedly rapid curing system for epoxy resins.

When employing the three component curing system, a major amount of thenitrogen-containing component and minor amounts of each of the other twocomponents are generally employed. However, this could be reversed sothat either of the hydroxyl containing or organo-rnetallic compound isin the majority. An excess of any of the three ingredients is notdetrimental to the adhes ve; 1t will merely serve as a filler. Asuitable composition of the three component cure system is onecontalning about .025 to about 500, preferably about 1 to about 25,parts of nitrogen-containing compound per part of hydroxyl containingcompound, and about .05 to about 5000, preferably about 1 to about 250,parts of n1trogen-contam1ng compound per part of organo-metalhccompound. Here, as elsewhere in the specification and claims, parts areby weight unless stated to the contrary.

In addition to the foregoing, the adhesive compos1t1on of this inventionmay include fillers and the lil se. Pan ticularly desirable is a finelydivided inorganic oxide such as titanium dioxide which has been found topromote sprayability in electrostatic equipment. Other suitablemorganicoxides are aluminum oxide (A1 and calcium oxide (CaO). Another highlydesirable component for inclusion in the adhesive of this invention is acorrosion inhibitive pigment such as a metal chromate (zinc, cadmium,calcium, strontium, lead, barium). By includmg a corrosion inhibitordirectly into the adhesive, it has been found unnecessary to separatelytreat the metal surface to be bonded with a corrosion inhibitive primer.

The substrates to be bonded may be treated with adhesion promotingprimers such as organosilanes to improve the bond strengths of theadhesive as is well known to the art.

The amount of the various components of the adhesive composition of thisinvention may vary over rather broad ranges. The nitrile rubber maysuitably be present to the extent of from about 6 to about 54 parts byweight per parts by weight of epoxy resin, with a range of 1540 parts byweight being preferred. The novolac epoxy resin may be present to theextent of from about 10 to about 300 parts by weight per hundred partsby weight of non-novolac epoxy resin with from 30 to 100 parts per 100parts of non-novolac epoxy resin being preferred. The curing system maysuitably be present in an amount of at least .15, and preferably atleast .8, amine hydrogen equivalents per epoxy equivalent. Higher levelsof curing agents are not detrimental as they serve merely as fillers,albeit expensive ones. The metal oxide and the corrosion inhibitivepigment may each be present to the extent of from 0 to about 36 parts byweight per 100 parts by weight of epoxy resin.

Spraying, fluidized beds, and vibrating beds represent automatedtechniques for applying the adhesive of this invention to the substrateto be bonded. Because of the powdery nature of the adhesive, it can bereadily removed by vacuum tools where desired. A preferred methodcomprises contacting a supply of powdered adhesive with a honeycomb corestructure which is at a temperature sufficiently high to cause theadhesive to transfer to the core edges (about -F.) and yet insufficientto activate the elevated temperature decomposable curing agent, removingthe adhesive coated core from the supply and, cooling the core structuresuch that the adhesive assumes a fused, non-tacky, adhering state.

The supply of powdered adhesive contacted by the heated core maysuitably be in the form of a flat bed or coating on a roller. Adhesionof the powder to the roll surface can be maintained by electrostaticforces, vacuum forces, friction such as found in a pile type fabric, orthe force of attraction which the roll surface inherently possesses withrespect to the powdered adhesive.

A typical honeycomb assembly to be bonded by the adhesive of thisinvention is illustrated in FIGS. 1-4 wherein:

FIG. 1 illustrates an airplane in which structural members are bondedwith the adhesive of this invention;

FIG. 2 is a cross-sectional view of the wing assembly taken along line2--2;

FIG. 3 is a sectional view taken along line 33 of FIG. 2; and

FIG. 4 is an enlarged fragmentary view of the bonded area of thehoneycomb core of FIG. 2.

Referring to FIGS. l-4, section 1 of a wing assembly 3 is shown. Thesection 1 comprises a doubler portion consisting of aluminum panels 5,7, and 9 bonded by adhesive layers 11 and 13. The honeycomb core 15 isbonded to the outer aluminum panels or skins 5 and 9 by means of anadhesive layer 17 and filleting adhesive 19 (see FIGS. 3 and 4). Theadhesive layer 17 and filleting adhesive 19 may be of the same ordiflerent compositions falling within the scope of the presentinvention. Preferably, adhesive layer 17 contains a corrosion inhibitivepigment. As particularly shown in FIGS. 3 and 4, the filleting adhesiveoccurs substantially exclusively along the edges 21 of the honeycombcells leaving the intra-cellular portion 23 open. Core sections arespliced together by means of an expandable adhesive 25 which may be inthe form of a tape or other suitable form. The channel close-out portion27 is also joined to the honeycomb core by means of an expandableadhesive 25.

To further illustrate the invention, the following nonlimiting examplesare provided in which all parts and percentages are by weight unlessotherwise expressed.

2 Trade designationEpon 1002. Trade designation ECN-1280.

The nitrile rubber is banded on a conventional rubber mill after whichthe epoxy resin and novolac resin, each in powder form, are added to therubber on the mill and 8 state. The parts to be bonded are thenassembled and the adhesive cured by subjecting the assemblage to atemperature of 250 F. and a pressure of 50 p.s.i. for 60 minutes in anautoclave.

(2) Fluidized bed.--The metal substrate such as a honeycomb core ispreheated to 150 F. and then dipped into a fluidized bed of the aboveadhesive. The adhesive fuses to the core edges, with the intra-cellulararea being free of adhesive. The desired assemblage is completed and theadhesive cured as above.

(3) Powder bed.-The metal substrate is preheated to 150 F. and thenbrought into contact with a pile fabric surfaced roller bearing a layerof loosely adhering powder. The heat fuses the adhesive particles to thesubstrate. Assembling and curing are accomplished as above.

r the mixture blended to a homogeneous mass. In successive order, thetitanium dioxide, dicyandiamide, and 3-p- EXAMPLES 242 chlorophenyl1,1-dimethyl urea are added. Durlng a dl- Adhesive compositions 1-11 areprepared in accordance tion of the latter ingredients, the rubber millis cooled with Example 1. Each adhesive is tested with the results withcirculating water to prevent heat buildup. After shown in Table 2.

TABLE 1 Epoxy Amine H Sample Epoxy resin 1 N itrile rubber Curing systemequiv. equiv.

1 Epon 100 11 Hycar 1072 36 Dicy 5 3.5 P-CPDMU 2 2. 25 191 ,178 2-- Epon840 65.1 d 26.5 D1cy 6.9 P-CPDMU 4.43 .384 .351 3 Epon 836 68.5 lo 27.0Dicy 7.32 P-CPDMU 4.72 .398 .373 4 26.8 Hycar1000 128 27.1 Dlcy 4.31P-CPDMU 2.78 .234 .219 5 11 Tylae 22lA 36 1316 1.... 3.5 P-CPDMU 2.25.191 .178 6" Iphy 6.57 P-CPDMU 2.78 .234 .151 11 1316 1.... 4. 45 MMPD31.94 .191 .227 Dicy 3.16 P-OPDMU 2.03 .169 .160 E 1 2 Dicy. 4.54 P-OPDMU2. 93 .251 .231 73.2 ECN1280 Dicy 3.28 PDMU4 5.46 234 .139 11 d0 89 ERLl0 Dicy 4.44 P-CPDMU 2,86 ,240 225 1 All parts are by weight.

2 Parachloro phenyl dimethylurea.

3 1,1-(4-methyl meta phenylene) bis 3,3-dimethylurea. 4 Phenyl dimethylurea.

5 Dicyandiamido.

' Isophthalyldihydrizide.

7 Tradename for a solid diglycidyl ether-bisphenol A resin.

8 Tradename for a viscous diglycidyl ether-bisphenol A resin.

9 Tradoname for a solid diglycidyl ether-bispheuol A resin.

" Tradename for a triglycidyl paraaminophenol resin.

Tradename for non-carboxylated acrylonitrile 1 butadiene copolymer TABLE2 0 .L.S. F. T-peel F. Honeycombpeel F.

B Overlap sheer strength in pounds/111. F Metal to metal peel strengthin pounds per inch width. Climbing drum sandwich 111 inch pounds perinch Width.

The resulting powdered adhesive may be stored indefinitely at 75 F.

The powdered adhesive is then applied to at least one of the twoelements to be bonded by one of the following methods:

(1) Electrostatic spraying.Employing an electrostatic spray gun such asa Ransburg REP hand gun, the powdered adhesive is electrostaticallycharged While passing through the gun. The powdered, electrostaticallycharged adhesive issuing from the gun is applied to an electricallygrounded metal sheet by electrostatic attraction. Excess adhesive isremoved by brushing or suction using a vacuum tool. The adhesive is thenfused to the metal surface by heating at 150 F. for 10 minutes in an aircirculating oven. At this stage, the adhesive is firmly adhered to thesurface in a non-tacky, uncured While the present invention has beenparticularly described with respect to aluminum substrates employed inaerospace structures, it is to be understood that the adhesive of thisinvention finds application to a variety of other substrates such asWood, steel, plastics and the like.

What is claimed is:

1. A solid, flowable, particulate, curable adhesive com- (a) at leastone curable epoxy resin having on the average more than one 1,2 epoxygroup per molecule,

(b) a copolymer derived from 1'8%46% by weight acrylonitrile, 55%82% byweight butacliene, and 0%15% by weight of a carboxylic acid, saidcopolymer being present to the extent of from about 6 to about 54 partsby weight per parts by weight of said epoxy resin, and

(c) an epoxy curing system present in an amount of at least 15 aminehydrogen equivalents per epoxy equivalent comprising at least one roomtemperature stable urea compound decomposable below 250 to provide atleast one active hydrogen-containing amine,

wherein said adhesive, after a 1 hour cure at 250 F., exhibits anoverlap shear value of at least 1500 p.s.i. in the range of 67 F. to 1802. The adhesive of claim 1 wherein said copolymer is derived from up to15% by weight of a carboxylic acid.

3. The adhesive of claim 1 wherein said curing system comprises amixture of 3-phenyl-1,1-dimethyl urea and dicyandiamide.

4. The adhesive of claim 1 wherein there are at least two epoxy resins,at least one of which is a condensation product of a polyhydric phenoland an epihalohydrin.

5. The adhesive of claim 1 wherein there are at least two epoxy resins,at least one of which is a condensation product of a polyhydric phenoland an epihalohydrin, and at least one of which is a novolac epoxy resinpresent to the extent of from about to about 300 parts by weight per 100parts by weight of said first recited epoxy resin.

'6. The composition of claim 1 wherein said curing system comprises atleast one room temperature stable urea compound decomposable below 250F. to provide at least one active hydrogen-containing amine, and atleast one compound selected from the class consisting of dicyandiamideand isophthalyldihydrazide.

7. The composition of claim 1 wherein said curing system comprises (1)at least one room temperature stable urea compound decomposable below250 F. to provide at least one active hydrogen-containing amine, (2) ahydroxyl containing organic compound, and (3) an organo lead or organomercury compound.

-8. A solid, flowable, particulate, curable adhesive comprising (a) atleast two epoxy resins, at least one of which is a condensation productof a polyhydric phenol and an epihalohydrin, and at least one of whichis a novolac epoxy resin present to the extent of from about 10 to about300 parts by weight per 100 parts by weight of said first recited epoxyresin,

(b) a copolymer derived from 18%46% by weight acrylonitrile, 55%-82% byweight butadiene, and up to by weight of a carboxylic acid, saidcopolymer being present to the extent of from about 6 to about 54 partsby weight per 100 parts by weight of said epoxy resin, and

(c) an epoxy curing system present in an amount of at least 15 aminehydrogen equivalents per epoxy equivalent comprising at least one roomtemperature stable urea compound decomposable below 250 to provide atleast one active hydrogen-containing amine,

wherein said adhesive, after a 1 hour cure at 250 F., exhibits anoverlap shear value of at least 1500 p.s.i. in the range of -67 F. to180 F.

9. A solid, flowable, particulate, curable adhesive comprising (a) atleast one curable epoxy resin having on the average more than one 1,2epoxy group per molecule,

(b) a copolymer derived from 18%-46% by weight acrylonitrile, 55%-82% byweight butadiene, and 0%l5% by weight of a carboxylic acid, saidcopolymer being present to the extent of from about 6 to about 54 partsby weight per parts by weight of said epoxy resin,

(0) an epoxy curing system present in an amount of 'at least 15 aminehydrogen equivalents per epoxy equivalent comprising at least one roomtempera- 'ture stable urea compound decomposable below 250 F. to provideat least one active hydrogen-cont'aining amine, and

(d) at least one metal compound selected from the class consisting of aninorganic metal oxide and inorganic metal chromate,

wherein said adhesive, after a 1 hour cure at 250 F., exhibits anoverlap shear value of at least 1500 p.s.i. in the range of 67 F. to F.

10. An article comprising a substrate bearing a coating of the adhesiveof claim 1.

11. An article comprising an aluminum substrate bearinga coating of theadhesive of claim 1.

12. An article comprising a honeycomb core substrate bearing a coatingof the adhesive of claim 1.

13. An article comprising a substrate bearing a coating of the adhesiveof claim 1 wherein there are at least two epoxy resins, at least one ofwhich is a condensation product of a polyhydric phenol and anepihalohydrin.

14. An article comprising a substrate bearing a coating of the adhesiveof claim 1 wherein there are at least two epoxy resins, at least one ofwhich is a condensation product of a polyhydric phenol and anepihalohydrin, and at least one of which is a novolac epoxy resinpresent to the extent of from about 10 to about 300 parts by weight per100 parts by weight of said first recited epoxy resin.

References Cited UNITED STATES PATENTS 2,879,252 3/1959 Been 260-8372,947,338 8/1960 Reid 260-836 3,100,160 8/1963 Korpman 260837 3,208,9809/1965 Gruver 260836 3,219,515 11/1965 Rice 260-836 3,312,754 4/1967Marks 260 -837 3,324,198 6/1967 Gruver 260-837 2,713,569 7/ 1955Greenlee 260-47 3,386,955 6/1968 Nawakowski 260-47 FOREIGN PATENTS999,383 7/1965 Great Britain 260-836 PAUL LIEBERMAN, Primary ExaminerU.S. Cl. X.R.

260-2 EC, 2 N, 37 Ep, 41.5 R, 47 EC, 47 EN, 830 TW, 836; 117-17, 37 R,132 BE, 132 CB, 138.8 A, 148, 68

